The present invention relates to time measurement apparatus and, in particular, to a system and method for measuring, with picosecond precision, intervals between single edged events, wherein each measured interval comprises the summation of a rough clock count and fine or calibrated vernier counts of measured fractional clock periods before and after each START and STOP event selected from a calibrated vernier memory.
Basic to the understanding of almost any physical event is a requirement of obtaining accurate timing information relative to the occurrence of the event. For a variety of developing technologies, it is critical to obtain measurements in the picosecond range (i.e. 1.0.times.10.sup.-12 seconds) with a related accuracy. Examples of such technologies are found in a host of research, testing and development applications from nuclear and materials research to semiconductor and device testing to radar, computer and communications systems developments.
Heretofore and although a variety of time measurement devices and methodologies have been used, measurement accuracy for non-repetitive events has been limited by the accuracy of the equipment's master clock. To date, the most accurate systems have been constructed around very precise crystal clocks operating at relatively high frequencies (e.g. 100 MHz) with nanosecond clock signal periods. Comparison of a measured event to such a clock provides a correspondingly accurate measurement of the sampled event, except for test fixture error and fractional clock periods lost when the beginning and ending of the event are not synchronous with the measurement clock. Where measured events are repetitive in nature, however, additional accuracy may be achieved over repetitive samplings by fitting the measured data via a variety of averaging or statistical smoothing or interpolation algorithms to obtain relatively precise measurements with minimal error.
For non-repetitive, single occurrence or asynchronous events, however, and especially events of sub-nanosecond duration, accuracy is critical since the uncorrected measurement errors may exceed or approach in magnitude the event being measured. That is, for most such systems, accuracy is obtainable only relative to complete clock cycles which are counted as the event is occurring and from which a time value is extracted. Where, however, the event begins or ends mid-cycle, the corresponding partial cycle intervals are lost and appear as error, over and above any inherent error in the system itself. This error may not be averaged.
Some systems of which Applicant is aware of which measure time relative to the counting of clock cycles from one or more oscillators may be found upon directing attention to U.S. Pat. Nos. 4,164,648; 4,186,298; 4,350,953; 4,397,031; and 4,598,375.
Applicant is also aware of a variety of attempts to expand the measurement scale or resolve or interpolate error occurring during measurement, which may be found upon directing attention to U.S. Pat. Nos. 2,896,160; 3,133,189; 3,218,553; 3,505,594; 3,753,111; 3,970,828; 4,165,459; 4,301,360; 4,504,155; and 4,613,950. These latter systems generally employ techniques for performing multiple levels of time measurement (e.g. a coarse count and a fine or vernier count representative of a fractional portion of a clock cycle). Simultaneous operation of the two counters or measurement device, depending upon the methodology employed, enables the measurement of the fractional cycle error.
Although the present invention uses a coarse clock counter to measure the full cycle portion of any event, it additionally uses a separate ramped vernier measurement means and a self or operator enabled, calibrated table look-up memory for independently measuring each fractional beginning and ending time interval relative to the base clock signals. Of fractional measurement apparatus of this type, Applicant is also aware of an article by R. Nutt, Digital Time Intervalometer, 39 Review of Scientific Instruments 1342 (September, 1968) and U.S. Pat. Nos. 4,303,983 and 4,637,733. Of these, U.S. Pat. No. 4,303,983 discloses a system operating to produce a coarse time determined from a number of base clock cycles counted during a synchronous interval portion and to which are added and subtracted fractional cycle times. The fractional times are determined from separate time amplitude conversion circuitry which is separately calibrated after each measurement via internally generated start/stop signals to produce conversion factors by which measured analog amplitudes are adjusted prior to being coupled to associated display apparatus. U.S. Pat. No. 4,637,733, in turn, discloses apparatus wherein a ramped linear voltage is used to determine the fractional beginning and end times of an asynchronous event. It particularly discloses a means for developing an error table for compensating for ramp non-linearity. The calibration table is determined through numerous samples of constant pulse duration, although of differing time separation. The error samples are averaged, with the average error values being stored for access during processing of measured events.